1. Field of the Invention
The present invention generally relates to a semiconductor device for protecting a secondary battery and a battery pack for a whole category of electronic equipment installed in computerized personal organizers, handsets, voice recognition devices, voice memory devices, computers, etc.
2. Discussion of the Background
Battery packs including at least one secondary battery are widely used in portable electronic equipment such as mobile phones and digital cameras for its ease of handling. As secondary batteries, large-capacity batteries, such as lithium ion batteries, lithium polymer batteries, and nickel metal hydride batteries, are generally used.
It is to be noted that, in the present specification, “current” represents an electrical current unless otherwise specified.
Because the amount of energy stored in such large-capacity batteries is extremely large, it can be degraded when charged excessively to an extremely high voltage, discharging electricity excessively, to have an extremely low voltage, and when receiving or outputting an overcurrent, which can lead to heat generation or even overheating to the point of ignition. (A value of the voltage remaining in the secondary battery below which the battery can be degraded is hereinafter referred to as an excessive-discharge voltage.)
Therefore, battery packs are generally provided with a protection circuit for preventing the secondary battery from being charged excessively, discharging electricity excessively, receiving or outputting overcurrent, outputting a short-circuit current, overheating, etc. In such cases, the protection circuit disconnects the secondary battery from a charger or a load, preventing the secondary battery from overheating and igniting as well as deteriorating.
Further, when the voltage of the secondary battery is below the excessive-discharge voltage, that is, extremely low (hereinafter “low voltage state”), connecting the battery pack to the charger is dangerous. More specifically, when the secondary battery is in the low voltage state, the secondary battery can be in an abnormal state, such as a short circuit. If the secondary battery in such an abnormal state is charged, the secondary battery will generate heat or be damaged, which might cause ignition thereof.
In view of the foregoing, a known battery pack is provided with a charge prevention circuit that prevents charging the secondary battery in the low voltage state.
Such a charge prevention circuit determines whether or not the secondary battery Bat is extremely low (hereinafter also “low-voltage detection level” or “low-voltage criterial voltage”) based on a particular reference voltage. Moreover, the low-voltage criterial voltage is preferably variable among multiple set values.
In a known charge prevention circuit, the low-voltage criterial voltage can be lowered by a voltage identical to a threshold voltage of an NMOS transistor included in the charge prevention circuit. However, the low-voltage criterial voltage cannot be raised.
Therefore, there is a need to be able to set the low-voltage criterial voltage for the secondary battery more flexibly.